1. Field of the Invention
The present invention is generally concerned with progressive multifocal ophthalmic lenses, meaning ophthalmic lenses in which the surface of one side (in practise usually that of the front side only) comprises an area in which the curvature varies progressively and continuously.
The invention is more particularly (but not exclusively) directed to what is in practise the most usual case, where this progressively variable curvature area lies between two constant curvature (and therefore part-spherical) areas of which one corresponds to the far vision area and the other corresponds to the near vision area.
2. Description of the Prior Art
As is known, the surface of a progressive multifocal ophthalmic lens which features in this way an area of progressively variable curvature, or progressive surface, is established point by point with reference to a spherical surface or reference spherical surface, each of its points being defined by an offset relative to the corresponding point on a reference spherical surface of this kind.
As is also known, a progressive surface of this kind is generally symmetrical to each side of a meridian line, hereinafter referred to for convenience as the main meridian, and it is only along this meridian and in a more or less extensive area to either side of it that the continuous variation of curvature may be obtained in a relatively strict manner without distortion of the images observed, although to either side of this main meridian there are lateral areas of aberration in which the images observed are more or less severely distorted.
From this point of view a progressive multifocal ophthalmic lens may be defined as one comprising a main meridian along at least part of which the curvature varies continuously and on either side of which the areas of aberration are symmetrical to each other.
In what follows, and for reasons of simplicity, the term "ophthalmic lens" will be used interchangeably for both the original blank, which has a circular contour, and the ophthalmic lens proper which is machined from a blank of this kind, its contour then reflecting that of the ring or surround of the eyeglass frame to which it is to be fitted.
Thus, from this point of view, the optical center of an ophthalmic lens is coincident with the geometrical center of the blank from which it is obtained.
It is known that on changing from far vision to near vision the eyes of a person converge so that the line of sight moves closer to the nose.
There have already been various proposals to obtain an acceptable degree of visual comfort for intermediate vision and near vision allowing for the gradual shifting towards the nose of the corresponding line of sight.
These provisions are summarized in French Pat. No. 1.509.090 of Nov. 29, 1966.
One of them involves turning the lens around its optical axis so that the main meridian passes both through the optical center of the lens and substantially through the point where the surface of the lens intersects the line of sight of the subject for near vision, as if, relative to the reference spherical surface, the progressive surface concerned were turned in its entirety through the corresponding angle about the optical center of the lens.
Although this arrangement proves satisfactory for intermediate vision, there results on the nose side a non-negligible incursion of the corresponding aberration area into the area of far vision, to the detriment of the extent of this.
Another known provision consists in offsetting the main meridian laterally towards the nose, to place it at a distance from the vertical line through the optical center of the lens chosen in practise so as to correspond to half the distance to this vertical line from the point where the surface of the lens intersects the line of sight of the subject for near vision, as if, relative to the reference spherical surface, the progressive surface concerned were itself offset as a whole by this amount relative to this vertical line.
Although this arrangement respects the integrity of the far vision area, it is hardly satisfactory for the intermediate vision area or for the near vision area, the line of sight of the subject never being equidistant from the lateral aberration areas or being so only at more or less one point, to the detriment of the required visual comfort.
The other proposals already put forward for improving visual comfort, especially that which is the subject of the French Pat. No. 1.509.090 mentioned hereinabove, although they have proven and may yet prove satisfactory, have the disadvantage of being relatively complex to implement.
A general objective of the present invention is an arrangement which provides a very simple way of procuring the required visual comfort.